Equilibria involving ions acids and bases
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Equilibria involving ions: acids and bases. AH Unit 2(b)(iii). Key question. What is are acids and bases?. Arrhenius definition. An acid is a substance that when added to water increases the concentration of H + (aq) ions . HA + (aq)  H + (aq) + A - (aq)

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Equilibria involving ions: acids and bases

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Equilibria involving ions acids and bases

Equilibria involving ions: acids and bases

AH Unit 2(b)(iii)


Key question

Key question

  • What is are acids and bases?


Arrhenius definition

Arrhenius definition

  • An acid is a substance that when added to water increases the concentration of H+(aq) ions.

    HA + (aq)  H+(aq) + A-(aq)

  • A base is a substance that when added to water increases the concentration of OH-(aq) ions.

    BOH + (aq)  B+(aq) + OH-(aq)


Key question1

Key question

  • What are the limitations of these definitions?


Bronsted lowry definitions

Bronsted-Lowry definitions

  • An acid is a proton donor.

    HA  H++ A-

    HA + H2O  H3O++ A-

  • A base is a proton acceptor.

    B + H+ BH+

    B + H3O+  BH+ + H2O


Hydronium ion

Hydronium ion


Conjugate acids and bases

Conjugate acids and bases


Ionisation of water

Ionisation of water

  • Water is amphoteric.

  • Can you write an expression for the equilibrium constant?


Ionic product of water

Ionic product of water

  • Kw has a value of 1 x 10-14 at 25ºC.

  • Known as the ionic product of water.

  • Value varies with temperature.


Ph scale

pH scale


Dissociation of acids

Dissociation of acids

This is a measure of the strength of an acid

- the larger the value of Ka, the stronger the acid.


Calculating ph of weak acids

Calculating pH of weak acids


Dissociation of bases

Dissociation of bases

This is a measure of the strength of a base

- the larger the value of Ka, the weaker the base.


Indicators

Indicators


Universal indicator

Universal indicator


Methyl orange

Methyl orange


Phenolphthalein

Phenolphthalein


Indicators1

Indicators

  • Are weak acids


Equilibria involving ions acids and bases

  • HIn and In- have different colours

  • Their ratio is dependant on [H3O+]

  • The colour of an indicator in any given solution therefore depends on the ratio, which in turn is determined by pH


Equilibria involving ions acids and bases

  • The theoretical point at which the colour change occurs is when [HIn] = [In-]

  • Therefore the colour change occurs when KIn = [H3O+]

  • pKIn = pH


Equilibria involving ions acids and bases

  • In practice, the colour change is not visible when [HIn] = [In-]

  • Instead, they must differ by a factor of 10

    • i.e. when [H+] = KIn± 10

    • OR when pH = pKIn ± 1


Choice of indicator

Choice of indicator

  • Colour change must occur as close to the equivalence point as possible.

  • Equivalence point – the point at which all of the acid has been exactly “neutralised” by all of the alkali.

  • Does this always occur at pH 7?


Methyl red

Methyl red


Phenolphthalein1

Phenolphthalein


Equilibria involving ions acids and bases

  • The colour chance must occur in the region of rapid pH change.

  • This means that the addition of half a drop of acid/base will cause a colour change.

  • The choice of indicator must therefore be made with reference to titration curves.


Strong acid strong base

Strong acid / strong base


Phenolphthalein2

Phenolphthalein


Examples

Examples


Practice

Practice


Buffers

Buffers


Buffer solutions

Buffer solutions

  • Is a solution where the pH remains approximately constant when small amounts of acid or bases are added.

  • Common examples:

    • blood

    • sea water


Acid buffers

Acid buffers

  • Consists of a weak acid with one of its salts (of a strong alkali)

    • e.g. ethanoic acid + sodium ethanoate

  • The acid is partially dissociated and equilibrium with its ions.

  • The salt is fully ionised.


Equilibria involving ions acids and bases

  • Addition of alkali:

Supplies H3O+(aq) ions if any removed in reacting with an added base.

  • Addition of acid:

CH3COONa(s) → Na+(aq) + CH3OO-(aq)

The conjugate base removes any added H+(aq)


Ph of buffer solutions

pH of buffer solutions

Because the by diluting a buffer the concentration of acid and salt will decrease in proportion, dilution will not affect the pH of a buffer solution.


Practice1

Practice


Basic buffers

Basic buffers

  • Consist of a weak base with one of its salts (of a strong acid).


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